1. Field of the Invention
The present invention relates to a semiconductor light emitting device having a multilayer structure comprising at least an n-type clad layer, an active layer, and a p-type clad layer each made of a II-VI group compound semiconductor, a method of producing the same, and an optical device provided with the same.
2. Description of the Related Art
In recent years, there is an increasing demand for high density and high resolution in recording and reproducing of data onto and from an optical disk or a magneto-optic disk. There are also attempts to develop a high-intensity display device, a low-loss optical fiber communication system, and an optical analysis instrument for analyzing DNA or special chemicals. Thus there is a need to develop a semiconductor light emitting device for use as a light source in these applications which is capable of emitting light with a color in the range of green and blue.
Promising candidates as materials used to form a semiconductor device capable of emitting light with a color in the range of green to blue are II-VI compound semiconductors containing at least one II-group element selected from the group consisting of zinc (Zn), magnesium (Mg), beryllium (Be), cadmium (Cd), mercury (Hg), and manganese (Mn) and at least one VI-group element selected from the group consisting of oxygen (O), sulfur (S), selenium (Se), and tellurium (Te). However, such a II-VI semiconductor light emitting device has a high Schottky barrier at interfaces between electrodes and semiconductors (refer to for example I. Suemune, Appl. Phys. Lett. 63 (1993) 2612), and thus has a high contact resistance which results in a high operating voltage. ZnSe is widely used as a layer in contact with an electrode. However, it is difficult to dope a p-type impurity into ZnSe to a high enough level to obtain a high enough carrier concentration. This makes it difficult to realize a good ohmic contact, and the result is a high operating voltage. As a result, high power dissipation occurs, which results in heat generation which in turn causes degradation of the device.
One technique proposed to avoid the above problem is to form an additional ZnTe layer on the ZnSe layer in such a manner that the ZnTe layer is doped with a p-type impurity to a higher level than that of the ZnSe layer so that the highly-doped ZnTe layer serves as a contact layer with the p-side electrode. However, if the ZnSe layer is grown directly on the ZnTe layer, a great valence-band discontinuity occurs at the interface between the ZnSe and ZnTe layers. This great valence-band discontinuity results in a high resistance which makes it impossible to achieve a low operating voltage. One technique proposed to solve the above problem is to employ a graded ZnSeTe layer in which the Se-Te composition ratio is gradually changed, thereby reducing the operating voltage (refer to for example Y. Fan, D. C. Grillo, M. D. Rinqle, J. Han, L. He, R. L. Gunshor, A. Salokatve, H. Jeon, M. Hovinen, A. V. Nurmikko, G. C. Hua and N. Otsuka, J. Vac. Sci. Technol. B12 (1994) 2480). Another technique proposed for the same purpose is to dispose a superlattice layer consisting of ZnSe and ZnTe between the ZnSe and ZnTe layers (refer to for example F. Hiei, M. Ikeda, M. Ozawa, T. Miyajima, A. Ishibashi and K. Akimoto, Electron. Lett., 29 (1993) 878; Japanese Patent Laid-Open No. 6-5920).
The above techniques in which a composition-graded layer or a superlattice layer is employed have the problem that it is difficult to form such a layer with high crystal quality. ZnSe has a lattice constant of 5.66942 xc3x85 while the lattice constant of ZnTe is 6.10 xc3x85. Such a large difference in the lattice constant causes introduction of misfit dislocations when a ZnTe layer is grown on a ZnSe layer. Misfit dislocations act as hole traps which cause a reduction in the concentration of p-type carriers which in turn results in an increase in the operating voltage. As a result, it is impossible to achieve a sufficient reduction in the power dissipation and thus it is impossible to avoid the degradation of the device to a sufficient degree.
In view of the above, it is an object of the present invention to provide a semiconductor light emitting device whose operating voltage can be easily reduced to a low enough level, a method of producing such a light emitting device, and an optical device provided with such a light emitting device.
According to an aspect of the invention, there is provided a semiconductor light emitting device comprising at least an n-type clad layer, an active layer, and a p-type clad layer formed into a multilayer structure using a II-VI compound semiconductor containing at least one II-group element selected from the group consisting of zinc, magnesium, beryllium, cadmium, manganese, and mercury and at least one VI-group element selected from the group consisting of oxygen, sulfur, selenium, and tellurium, the semiconductor light emitting device also comprising a p-side electrode electrically connected to the p-type clad layer, the semiconductor light emitting device being characterized in that a contact layer is provided between the p-type clad layer and the p-side electrode, the contact layer being formed, at least in part, of a II-VI group semiconductor containing an alkali metal element serving as a p-type impurity.
According to another aspect of the invention, there is provided a semiconductor light emitting device comprising at least an n-type clad layer, an active layer, and a p-type clad layer formed into a multilayer structure using a II-VI compound semiconductor containing at least one II-group element selected from the group consisting of zinc, magnesium, beryllium, cadmium, manganese, and mercury and at least one VI-group element selected from the group consisting of oxygen, sulfur, selenium, and tellurium, the semiconductor light emitting device also comprising a p-side electrode electrically connected to the p-type clad layer, wherein there is provided a contact layer between the p-type clad layer and the p-side electrode, the contact layer containing a product of thermal reaction between an alkali compound and a II-VI group compound semiconductor, or containing an alkali compound and a product of thermal reaction between an alkali compound and a II-VI group compound semiconductor.
According to still another aspect of the invention, there is provided a method of producing a semiconductor light emitting device comprising at least an n-type clad layer, an active layer, and a p-type clad layer formed into a multilayer structure using a II-VI compound semiconductor containing at least one II-group element selected from the group consisting of zinc, magnesium, beryllium, cadmium, manganese, and mercury and at least one VI-group element selected from the group consisting of oxygen, sulfur, selenium, and tellurium, the method comprising the steps of: forming a plurality of II-VI group compound semiconductor layers into a multilayer structure, the semiconductor layers including at least the n-type clad layer, the active layer, and the p-type clad layer; forming an alkali compound layer on the surface of a II-VI group compound layer located on the side, adjacent to the p-type clad layer, of the active layer; after said step of forming the alkali compound layer, forming a p-side electrode in an area corresponding to the alkali compound layer;
According to still another aspect of the invention, there is provided an optical device including a semiconductor light emitting device, the semiconductor light emitting device comprising at least an n-type clad layer, an active layer, and a p-type clad layer formed into a multilayer structure using a II-VI compound semiconductor containing at least one II-group element selected from the group consisting of zinc, magnesium, beryllium, cadmium, manganese, and mercury and at least one VI-group element selected from the group consisting of oxygen, sulfur, selenium, and tellurium, the semiconductor light emitting device also comprising a p-side electrode electrically connected to the p-type clad layer, wherein a contact layer is provided between the p-type clad layer and the p-side electrode, the contact layer being formed, at least in part, of a II-VI group semiconductor containing an alkali metal element serving as a p-type impurity.
According to still another aspect of the invention, there is provided an optical device including a semiconductor light emitting device, the semiconductor light emitting device comprising at least an n-type clad layer, an active layer, and a p-type clad layer formed into a multilayer structure using a II-VI compound semiconductor containing at least one II-group element selected from the group consisting of zinc, magnesium, beryllium, cadmium, manganese, and mercury and at least one VI-group element selected from the group consisting of oxygen, sulfur, selenium, and tellurium, the semiconductor light emitting device also comprising a p-side electrode electrically connected to the p-type clad layer, wherein there is provided a contact layer between the p-type clad layer and the p-side electrode, the contact layer containing a product of thermal reaction between an alkali compound and a II-VI group compound semiconductor, or containing an alkali compound and a product of thermal reaction between an alkali compound and a II-VI group compound semiconductor.
In the semiconductor light emitting device according to the present invention, when a voltage is applied between the n-side electrode and the p-side electrode, a current is injected into the active layer via the contact layer. As a result of the injection of the current, emission of light occurs in the active layer. Because the contact layer is doped with an alkali metal element so that it has a low electric resistance, the electric resistance at the interface between the p-side electrode and the contact layer becomes low and thus the voltage drop across the interface also becomes low. As a result, the semiconductor light emitting device can be operated with low electric power without generating a significantly great amount of heat, and thus a long device life can be achieved.
In the semiconductor light emitting device according to another aspect of the present invention, when a voltage is applied between the n-side electrode and the p-side electrode, a current is injected into the active layer via the contact layer. As a result of the injection of the current, emission of light occurs in the active layer. Because the contact layer is formed to have a low electric resistance by means of incorporating a product of thermal reaction between an alkali compound and a II-VI group compound semiconductor, the electric resistance at the interface between the p-side electrode and the contact layer becomes low and thus the semiconductor light emitting device can be operated with low electric power.
In the semiconductor light emitting device according to still another aspect of the present invention, a plurality of II-VI group compound layers including at least an n-type clad layer, an active layer, and a p-type clad layer are formed into a multilayer structure, and then an alkali compound layer is formed on the surface of a II-VI group compound layer located on the side, adjacent to the p-type clad layer, of the active layer. After that, a p-side electrode is formed in an area corresponding to the alkali compound layer.
Furthermore, since the optical device according to the invention has a semiconductor light emitting device according to the invention, the semiconductor light emitting device can be operated such that light is emitted in its active layer by applying a small voltage.